T&D World Magazine

Special Plan to Rebuild 115-kV Transmission Line Keeps Lines Energized Most of the Time

In mid-December Henkels & McCoy completed Phase One of the Plumtree to Triangle 115-kV Rebuild Project in western Connecticut for Northeast Utilities. The project is replacing 16, 115-kV lattice steel towers with 32 steel monopole structures. New conductor and OPGW (optical ground wire) over 1.8 miles of right of way (ROW) will be strung. Two new monopoles will replace each lattice tower. For the duration of the project, keeping the lines energized except for those times when it is absolutely necessary to have an outage to perform the work has presented a unique challenge. The lines had to be de-energized, for example, when conductors were moved from the old structure to the new poles, and when new conductor was being pulled in. But most of the time when the work was being done the lines had to remain energized.

The original lattice steel structures held three separate 115-kV circuits: Two circuits on the outer arms and one circuit in the center space. The rebuild project will replace these circuits with just two circuits comprised of heavier conductors which will be mounted one each on the two new rows of monopole towers. The original conductors were 397.5 kcmil ACSR and 556.5 kcmil ACSR. The new conductors will be 1272 kcmil ACSR. According to Henkels & McCoy construction manager Danny Ashmore, each lattice structure will be replaced by two steel poles: one slightly to the north of the existing structure on the same ROW and one slightly to the south of the existing structure.

Work started in July 2006, first with construction of the tower foundations on the north and south sides of existing structures; then with installation of the monopole towers on the north side only, all with the lines energized. Ashmore said, “In the final configuration there will be two monopoles with arms tipped in toward the existing lattice structure which will then be removed.”

To arrive at that final arrangement, however, a temporary line was needed. As Ashmore explains: “We built the north side towers with arms going out in both directions. Then we installed a temporary line on the furthest outside arms. Next we transferred the existing conductor from the lattice structure to the inside arms of the tower and removed the lattice structure arms (see photo). For safety reasons this move was done with the line de-energized. Now with the two lines installed on the tower, we can remove the center and south side lines from the lattice structures and dismantle them. In Phase Two, we will build the south side monopole towers, install the south side lines, and then remove the temporary lines on the outside of the north towers.”

Safety is a prime concern for the contractor on this project, so potential safety problems and danger areas were identified and appropriate work procedures developed. In addition, Ashmore explained, “We have a special safety plan with respect to the demolition of the lattice structures because of constraints such as terrain and the fact that the line traverses residential neighborhoods.”

Work is expected to be complete by spring 2007.

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